Chromosomal Alterations Questions and Answers

Answer: D

Translocation was first discovered in Drosophila by Bridges (1923). Calvin Bridges studied chromosome rearrangements in the fruit fly.

Morgan worked on linkage and sex-linked inheritance, Mendel on the laws of inheritance in peas, and Darwin on evolution, so they are not credited with translocation.

Bridges worked with Morgan's group on Drosophila and described chromosome rearrangements including translocation in the early 1920s. He is also known for the discovery of non-disjunction.

Answer: A

Nullisomy is the condition in which a pair of chromosomes is missing from the somatic complement, written 2n - 2. The organism lacks both members of one pair.

Monosomy (2n - 1) lacks one chromosome only, trisomy (2n + 1) has one extra, and tetrasomy (2n + 2) has an extra pair.

The term for a chromosome number 2n - 2 is nullisomic. Nullisomics are often not viable in diploids but are found in polyploids such as wheat, where other chromosomes can make up for the missing pair.

Answer: C

The key's answer is Polyploidy.

A change in the number of chromosomes is a numerical aberration. Adding or losing a whole set of chromosomes is euploidy, of which polyploidy (three or more sets) is the main form, while adding or losing single chromosomes is aneuploidy. Of the options offered, polyploidy is the only change in chromosome number.

Translocation, inversion and duplication are structural aberrations: they rearrange, reverse or repeat segments of chromosomes without changing their number.

Answer: B

The phenomenon is non-disjunction.

Bridges' genic balance theory says sex in Drosophila depends on the balance between X chromosomes and sets of autosomes. Non-disjunction, the failure of chromosomes to separate at meiosis, upsets that balance by producing gametes with extra or missing X chromosomes. The resulting flies, such as XXY females, XO males, and the intersexes and metafemales from triploid lines, change the X:A ratio and the sex that follows; studying them is how Bridges worked the theory out.

Linkage and crossing over rearrange alleles but do not change chromosome numbers, and sex in Drosophila is genetic, not environmental.

Answer: C

Down's syndrome is caused by an aberration in chromosome 21.

Most cases are trisomy 21, three copies of chromosome 21, usually from non-disjunction in the mother's meiosis; a few arise from a Robertsonian translocation carrying extra chromosome 21 material, or from mosaicism. The features include intellectual disability, a flat face and upward-slanting eyes, a single palmar crease and heart defects.

No viable syndrome involves trisomy of chromosome 2. Chromosome 20 is not the cause, and X-chromosome aneuploidies give Turner (XO) or Klinefelter (XXY) syndrome.

Answer: C

Chromosomal elimination is a strategy to develop haploids.

In the bulbosum method, barley (Hordeum vulgare) is crossed with Hordeum bulbosum. After fertilisation the bulbosum chromosomes are progressively eliminated from the embryo, leaving a haploid with only the barley set; the embryo is rescued by culture. The wheat x maize cross works the same way. Doubling the haploid gives a fully homozygous line in one step.

Polyploids, amphidiploids and autopolyploids involve adding chromosome sets, the opposite of elimination.

Answer: C

The ‘A’ genome came from Triticum monococcum.

Bread wheat, Triticum aestivum, is a hexaploid with the genome formula AABBDD. Its A genome comes from the diploid einkorn group: textbooks name T. monococcum, while modern studies point to its close relative T. urartu. Either way, among the options it is T. monococcum.

Aegilops tauschii, also called Ae. squarrosa (two names for one species), contributed the D genome. Ae. speltoides is closest to the B genome donor.

Answer: B

The other parent is B. campestris.

U's triangle shows how three diploid Brassica species combine to form three amphidiploids. B. juncea (Indian mustard, AABB, 2n = 36) arose from B. campestris (now B. rapa, AA, 2n = 20) crossed with B. nigra (BB, 2n = 16).

B. oleracea is the third diploid (CC); it is a parent of B. napus (with B. campestris) and of B. carinata (with B. nigra). B. napus and B. carinata are themselves amphidiploids, not parents of B. juncea.

Answer: A

The aberration is a paracentric inversion.

In a paracentric inversion the inverted segment does not include the centromere. In a heterozygote the chromosomes pair by forming an inversion loop, and a single crossover inside the loop produces one chromatid with two centromeres (dicentric) and one with none (acentric). At anaphase I the dicentric forms a bridge that breaks, and the acentric fragment is lost.

A crossover in a pericentric inversion, which includes the centromere, gives chromatids with duplications and deletions but each with one centromere. Reciprocal translocations and simple deletions do not routinely give this dicentric and acentric pair.

Answer: C

The induced tetraploid was sterile because of problems in meiosis.

Colchicine blocks spindle formation, so the chromosomes double without the cell dividing. An autotetraploid has four homologous copies of each chromosome, and at meiosis these can pair as quadrivalents, trivalents plus univalents, or bivalents in various combinations. Irregular segregation then produces gametes with unbalanced chromosome numbers (aneuploid gametes), many of which are not viable, so fertility falls sharply.

Mitosis is not the problem: the tetraploid plant grows normally, and indeed its cells are often larger. Male sterility and self-incompatibility are specific genetic systems controlling pollen function or self-pollination; they are not the general cause of sterility in newly induced polyploids.